This invention relates to the formation of two or more toner images in a single frame or area of an image member. Although not limited thereto, it is particularly usable in forming accent color images on a single frame of an image member.
U.S. Pat. No. 5,001,028 to Mosehauer et al is representative of a number of references describing a process in which a photoconductive image member is uniformly charged and imagewise exposed to create an electrostatic image. Toner is applied to the electrostatic image to create a toner image. Usually, in this process, discharged area development is used. Thus, the toner applied is of the same polarity as the electrostatic image. It deposits in the areas of lowest charge (the discharged areas) to form a toner image having a density which is greatest in the portions of the image receiving the greatest exposure.
Although not absolutely necessary in this process, the image member is, again, uniformly charged with a charge of the same polarity as the original charge. It is, again, imagewise exposed to form a second electrostatic image, generally in the portions of the image member not covered by the first toner image. The second electrostatic image is toned, again with a toner of the same polarity as the charge to create a second toner image. The process can be repeated with a third electrostatic image toned by a third color toner to create a three color image, etc. The two (or more) color image is transferred in a single step to a receiving sheet and fused also in a single step.
Although the process is not necessarily limited to such applications, it is most commonly used to provide accent color prints or copies with laser or LED printhead electronic exposure. All commercial applications known to us use electronic exposure and discharged area development.
The process has a number of advantages in accent color applications. It eliminates the troublesome and expensive steps usually used in registering images at transfer. If it uses separate exposure stations, it can produce accent color output at the same speed as single color output.
It is important that the second and subsequent toning steps not disturb the first toner image. Otherwise, toner from the first toner image gets mixed into the second development station ("scavenging") and toner from the second development station can be deposited on the first toner image ("overtoning"). Recharging between images reduces overtoning. Much of the art prior to Mosehauer recommends use of projection toning for the second and subsequent toning steps in order not to disturb the first image. The Mosehauer patent suggests that excellent results are obtained using a high coercivity carrier in a two component magnetic brush having a rotating magnetic core. The Mosehauer approach provides high density images at high process speed with less color mixing than other high density, high speed systems.
U.S. Pat. No. 4,778,740 (Matsushita) notes a problem observed in such systems that when the second electrostatic image includes discharged areas immediately next to the first toner image, the first toner has a tendency to migrate into the second image. The solution suggested is to leave a one pixel gap from the first image in the second exposure. This can be accomplished in an electronic exposure system providing registration between the two exposures is very accurate. However, it requires excellent registration and leaves a thin, untoned area between the two images which can show up as a white streak or "halo".
U.S. Pat. No. 5,025,292 shows a system, used commercially, in which a series of color separation images are formed on a single image member using liquid developing. The first image is heated to dry it so it is fixed during the second exposure. The second exposure is made through a transparent support in order to create an electrostatic image that, in fact, overlaps the first color image, which electrostatic image is toned to form overlapping color toner images. This process is also carried out with discharged area development and the image member is recharged between images.
Many prior patents show exposure through a base of a photoconductive member for various purposes, usually associated with using the same magnetic brush to both clean and develop or trying to expose and develop at the same time. See, for example, U.S. Pat. Nos. 3,703,335; 5,159,389; and 5,053,821.